1. Field of the Invention
The present invention relates to a system for arranging the operations and connections between a computer or a similar device which belongs to a process and a control station needed by the process operator for communicating with the computer or, if the computer breaks down, for linking these devices to the process and controlling it.
2. Description of the Prior Art
The conventional present-day systems in which a process is controlled by a computer have several deficiencies. Because of the relatively great unreliability of computers, stand-by controllers and back-up stations in which the control signals given by the computer are transformed into a form suitable for the actuators of the process and which can also work as reserve devices when the computer breaks down are normally used. These devices considerably increase the price of the control system. Another deficiency is that, to identify the measuring and control points, a code consisting of letters and numbers is used which the process operator must remember in order to obtain the data corresponding to a given measurement and control point from the computer. Thirdly, if the computer breaks down, the person responsible for process control may have to use controls which he need not use when the computer is in order, and therefore he cannot use them well enough. In addition, several computer-based process control systems have characteristics which are not at all necessary in all types of processes.
Consequently, it is desirable to eliminate the above disadvantages and to provide a system by which a process can be controlled simply when, for example, a computer breaks down and in which a separate stand-by controller or back-up station for manual process control is not needed for each actuator.